THE HYPOBLAST. 



the epiblast, and causes in it a slight indentation. The hypoblast in the 

 region of the ridge is formed of two layers of cells, the ridge being entirely 

 due to the uppermost of the two. 



In sections in front of 

 this a cylindrical rod, which 

 can at once be recognized 

 as the notochord, and is 

 continuous with the ridge 

 just described, begins to be 

 split off from the hypoblast 

 (fig. 25 A, Ch}. It is diffi- 

 cult to say at what point 

 the separation of this rod 

 from the hypoblast is com- 

 pleted, since all intermedi- 

 ate gradations between 

 complete separation and 

 complete attachment are to 

 be seen. 



Shortly after the separ- 

 ation takes place, a fairly 

 thick bridge is found con- 

 necting the two lateral 

 halves of the hypoblast, 

 but this bridge is anterior- 

 ly excessively delicate and 

 thin, and in some cases is 



FIG. 25. THREE SECTIONS OF A PRISTIURUS 

 EMBRYO SLIGHTLY OLDER THAN FIG. 28 B. 



The sections shew the development of the noto- 

 chord. 



Ch. notochord ; Ch'. developing notochord ; mg. 



medullary groove ; Ip. lateral plate of mesoblast ; 



ep. epiblast ; hy, hypoblast. 

 barely visible except with 



high powers. In some sections I have observed possible indications of the 

 process like that described by Calberla for Petronyzon, by which the lateral 

 parts of the hypoblast grow in underneath the axial part, and so isolate it 

 bodily as the notochord. 



It is not absolutely clear whether the notochord is to be 

 regarded as an axial differentiation of the hypoblast, or as an 

 axial differentiation of the lower layer cells. 



The facts of development both in Amphioxus and Elasmo- 

 branchii tend towards the former view ; but the nearly simul- 

 taneous differentiation of the notochord and the mesoblastic 

 plates lends some support to the supposition that the notochord 

 may be merely a median plate of mesoblast developed slightly 

 later than the two lateral plates. 



The alimentary canal or mesenteron was left as a space 

 between the hypoblast and the yolk, ending blindly in front, but 



